In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence. In a basic structure of such a light-emitting element, a substance having a light-emitting property is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the substance having a light-emitting property.
Since such a light-emitting element is of self-light-emitting type, it is considered that the light-emitting element has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not required, and so on and is therefore suitable as flat panel display elements. In addition, other advantages of such a light-emitting element are that the element can be manufactured to be thin and lightweight and the response speed is very high.
Since the light-emitting element can be formed into a film shape, surface light emission can be easily obtained by forming a large-area element. This is a feature which is difficult to be obtained by point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Accordingly, the light-emitting element is extremely effective for use as a surface light source applicable to illumination and the like.
Light-emitting elements using electroluminescence are classified broadly according to whether they use an organic compound or an inorganic compound as a substance having a light-emitting property.
When an organic compound is used as a light-emitting substance, electrons and holes are injected into a layer including a light-emitting organic compound from a pair of electrodes by voltage application to a light-emitting element, so that a current flows therethrough. The electrons and holes (i.e., carriers) are recombined, and thus, the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Based on this mechanism, such a light-emitting element is called current excitation type light-emitting element.
It is to be noted that the excited state generated by an organic compound can be a singlet excited state or a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
In improving element characteristics of such a light-emitting element, there are a lot of problems which depend on a material used, and in order to solve the problems, improvement of an element structure, development of a material, and the like have been carried out.
For example, Reference 1 describes a light-emitting element in which a guest substance is dispersed in the whole organic function layer between electrodes to enhance the possibility of injection and recombination for electrons and holes. However, Reference 1 does not refer to a lifetime of the light-emitting element.
Such a light-emitting element including a light-emitting organic compound can be driven at lower voltage than a light-emitting element using an inorganic compound; however, such a light-emitting element including a light-emitting organic compound has a problem in that its lifetime is short. Thus, it is desirable that such a light-emitting element has a longer lifetime (Reference 1: Japanese Published Patent Application No. H 6-158038).